The main functions of spacers in oil filled transformers are to mechanically separate and support windings. Typically they are also stressed electrically with an AC electrical field and a high impulse electric field in testing, which is often dimensioning for the spacer thickness.
When transformer designs are optimized for maximum compactness the spacer ability to accept a high dielectric stress becomes vital. The allowed voltage between coils in transformers is often limited by the initiation of a breakdown outside the spacer and along the spacer-oil interface.
This effect occurs primarily as a result of the different dielectric constants of typical spacer materials and transformer oil. When a higher dielectric constant material like pressboard and transformer oil meet at a conductor, the electric field in the oil wedge is enhanced by a factor approximately equal to the ratio of dielectric constants, or 4.5/2.2=approximately 2 in the pressboard-oil case. There are several geometric ways that this field enhancement can occur.
Where a rounded spacer is in contact with the conductor, an oil wedge occurs in the contact area of the spacer and the conductor. The electric field in this arrangement increases at the contact area. The field in the contact area is approximately twice the average field away from the conductor. It is also known that the interface along the spacers is a weak point and that electric breakdowns preferable occur in the vicinity of the spacers. The oil volume exposed to this field enhancement depends on the geometry of the spacer, and is normally quite small.
Another critical area is where rounded conductors and spacers comes into contact with spacers which are arranged perpendicular to the conductors.
This oil wedge is present along the conductor on all turns of the transformer and consequently has a quite large volume and consequently a larger probability for triggering a discharge during impulse testing. Such a discharge created between the spacer and the conductor is probably not too dangerous if it happens far from the edges of the spacers, but if it happens close to the spacer edge there should be a substantial risk that the discharge propagates along the spacer-oil interface to the next winding layer, causing a breakdown. The observation in real testing is also that breakdown preferentially does occur at spacers.
Still another critical area is where an axial spacer, conductor corner and a radial spacer meet. At the outermost turn of a disc winding the conductor meets an axial pressboard spacer, which defines the distance to the next barrier. This barrier is followed by a further spacer, a new barrier etc. The result is a similar field enhancement at the axial spacer oil wedge, and a combined axial and radial field enhancement occurs at the outer conductor edge. This is the most vulnerable part of the winding, with the highest failure probability.
The present invention seeks to provide an improved oil filled power transformer and improved spacers getting improved breakdown strength of the transformer.